Method for electrolytic production of chlorine

ABSTRACT

1,128, 804. Electrolytic production of chlorine and hypochlorite. E. H. B. NYSTROEM. Nov. 5, 1965 [Dec.3, 1964; Dec.4, 1964], Nos.49300/64 and 49364/64. Heading C7B. Chlorine is produced by electrolysis of an aqueous chloride solution at a pressure not exceeding about 200 mm. Hg. Typically sodium or potassium chloride solution is continuously introduced into the cell and chlorine is continuously removed. Chlorine so removed or an aqueous solution of active chlorine (hypochlorite) produced by such electrolysis is added to water to be chlorinated at a rate depending on a determined property of water which has been previously chlorinated, the amount of chlorine or active chlorine solution being controlled by varying the electrolysis current. Typically in this case the electrolysis is carried out at a pressure of 1 to 10 mm. Hg. obtained by use of a jet pump employing as the jet a stream of the water to be chlorinated, the pump also removing electrolysis products from the cell. The electrolysis cell may be a tubular cathode surrounding an anode or may be of a rectangular vertical stack construction held together by I-bolts, the stack consisting in order upwardly of a plastic plate, an apertured packing piece, a graphite anode, a further apertured packing piece, a plastic frame, another apertured packing piece and a further plastic plate, the cathode of perforated iron (or an iron-containing compound) being disposed close to the anode in a holder depending from the second plastic plate. The cell temperature may be held below a value at which side-reactions occur by allowing boiling, this occurring at a temperature dependent on the pressure, and cooling tubes may also be provided.

United States Patent 3,520,787 METHOD FOR ELECTROLYTIC PRODUCTION OFCHLORINE Ernst Holger Berti] Nystroem, 11 Vasavaegen Djursholm 2,Stockholm, Sweden No Drawing. Filed Oct. 19, 1965, Ser. No. 498,152Claims priority, application Great Britain, Dec. 3, 1964, 49,300/ 64;Dec. v4, 1964, 49,364/64 Int. Cl. Blllk l /00; C01b 7/02 US. Cl. 204-944"Claims ABSTRACT OF THE DISCLOSURE This invention relates to a methodof electrolysis and to an electrolytic cell for carrying out the methodof the invention.

In the electrolytic production of chlorine and hypochlorite, from whichalso chlorates are produced, very many different constructions of cellshave been made. The fundamental idea has been to keep theelectrolytically produced products apart and for that reason someconstructions have a membrane between the electrodes. Others havemercury as cathode so that the mercury will maintain the produced alkalimetal. The reasons for these different constructions has mostly been toavoid the formed chlorine, which is to be converted to hypochlorite,being reduced to chloride by the action of the atomic hydrogen producedat the cathode.

However, when it is desired to produce hypochlorite and chlorate, cellsare used in which the products from the anode and the cathode are mixedwith each other. In these cells it is not possible to convert more thana small part of the salt in the electrolyte to chlorine, hypochlorite orchlorates because of undesired side-reactions. These side-reactionsinclude reactions of the abovementioned type where reduction of theformed chlorine to chloride takes place and other reactions where, forexample, oxygen is formed, e.g. from further electrolysis of theproduced hypochlorite. This phenomenon is well known and is described inthe literature.

In the above cells it has also been necessary to keep the temperaturedown since otherwise the above sidereactions are assisted. Temperaturecontrol has been carried out by using, for example, cooling tubes butthis has not always proved satisfactory.

The present invention provides a method for the electrolytic productionof chlorine, hypochlorite and chlorates where the above disadvantageshave been substantially eliminated. The invention also includes anapparatus for carrying out the above method.

According to the present invention there is provided a method ofelectrolysis in which the electrolysis is carried out at reducedpressure.

More particularly there is provided a method for the electrolyticproduction of chlorine in which the electrolysis is carried out at lowerpressure.

The reduced pressure may be chosen so that the temperature of theelectrolyte is maintained at or below a certain value. The pressure iscontrolled so that if the temperature exceeds a desired level theelectrolyte starts to boil. The evaporation then cools the electrolytedown 3,520,77 Patented July 14, 1970 to below the required temperature;until it does this the temperature is maintained at boiling.

The present invention also includes cells adapted to operate at reducedpressure. It should be emphasized that these cells may be provided withconventional cooling means if desired.

Operating the cell at low pressure means that the chlorine produced atthe anode only dissolves in the electrolyte to a very small amount, therest evaporates. This amount is still smaller where the electrolyte iskept at the boiling point and especially when the electrolyte is underboiling. This low concentration of chlorine will form a low amount ofhypochlorite and chlorate, that is, very little side-reaction will takeplace. In addition the electrode-material will last longer as there willbe only a small concentration of chlorine and consequently but littlecorrosion. This is especially the case regarding porous material such asgraphite or ferrite.

The method, and the cells, according to the present invention may beused continuously or intermittently. In the first case the low pressurein the cell can be used to draw the electrolyte into the cell. As thishappens a gas phase and a liquid phase are drawn out of the cell. It ispossible either to keep these phases apart or to mix them, as desired.In the gas phase the chlorine and the hydrogen are mixed and can beseparated from each other in known manner, or if hydrochloric acid is tobe produced, then the mixture can be directly burned to that. Where thegas phase and the liquid phase from the cell are mixed with each otherthen the chlorine and the produced alkali are in stoichiometric balancewith each other and form hypochlorite which can be used directly or canbe converted to chlorate or perchlorate.

The present invention also concerns a method for the continuoustreatment of water with chlorine and/or an aqueous solution of activechlorine, preferably in free and/or hypochlorite-bound form, thechlorine or the aqueous solution being supplied to the water in apredetermined amount. The amount of chlorine to be added to an amount ofwater may be assessed by determining a convenient property of a furtheramount of water chlorinate dsubstantially immediately before. Theinvention also encompasses an apparatus for carrying out the method.

Processes for the chlorination of water are known. The best known ofthem is the chlorination of water to obtain drinking-water. In thisconnection, it has been common practice to buy liquid chlorine from achlorinealkali plant and to transport it, under pressure in pressureresistant containers, to the place of use. The gaseous chlorine is thencontinuously dosed from the container into a stream of raw water at asufiicient rate to give the water a content of active chlorinesufiicient, in view of its content of bacteria and viruses, fordisinfection of the water. As however this method often gives fault inthe dosing it has been necessary to maintain the concentration ofchlorine higher than what is necessary for attaining disinfection. This,however, results in an increased corrosion in water conduits, impairedtaste of the drinking-water and risk of poisoning plants by wateringthem therewith. Further, a very slight excess of chlorine in waterresults in considerable additional costs for the water purification.

One known method of chlorinating water comprises producing the chlorineor the aqueous solution of active chlorine by electrolysis at the placeof treating the water, the chlorine being added in a continuous processin an amount depending on a determined property of a bath of waterpreviously chlorinated. However, in the above method, apparatus with aseparating membrane between the two electrodes is used, and theconstruction and also the handling is very complicated. On the otherhand, if the electrolytic cell is used without any membrane between theelectrodes then the formed chlorine and hypochlorite will partly bereduced at the cathode by the atomic hydrogen formed there. It ispossible to cut down this reduction but only to a degree where the lowchlorine production, in comparison with the theoretical production, isstill very low.

The object of the present invention is to avoid these disadvantages.

According to the present invention there is provided a method ofchlorinating Water which comprises producing the chlorine gas or theaqueous solution of active chlorine by electrolysis of a chloridesolution at the place of treating the water, the chlorine being addedcontinuously in an amount depending on a determined property of 'waterpreviously chlorinated, said amount being controlled by varying thecurrent for electrolysis, and said electrolytic cell being kept atreduced pressure during the electrolysis.

The vacuum pumps used to evacuate the cells according to the presentinvention may be of any type. In many cases it may be useful to protectthe electrolyte with a scrubber or wash placed between the cell and thepump. Between the cell and the scrubber or wash may then be placed oneor two separators to take away the gases and/or the liquid. Very oftenit has been useful to have as vacuum pump a jet pump where the jetstream consists of products from the cell pumped in circulation. This isespecially the case when the chlorine is to be dissolved into thedeparting electrolyte. In other cases it has been useful to use aliquid-piston-type rotating blower using the departing electrolyte asliquid.

In the embodiment of the invention in which the chlorine produced isused to treat water, a preferred pump for producing the low pressure inthe cell is a jet pump. Such a pump may use a stream of water to bechlorinated as the jet. The gaseous contents of the cell are extractedby this jet and the resulting solution may be fed into a further amountof water to be chlorinated. The vacuum can also be created by a pumpthrough which the liquid from the electrolytic cell as well as the gasesare drawn. Then for instance a liquid-piston-type rotary blower can beused where the liquid at least partly consists of the products from theelectrolytic cell. Products from the pump are mixed with the water whichis to be chlorinated.

It is of importance to have a low pressure throughout the methodaccording to the present invention. The pressure used is determineddepending on the desired temperature. It may be useful to have a highdegree of vacuum such as under 1 mm. Hg. Preferred pressures are howeverbetween 1 mm. Hg and mm. Hg, although pressures between 10 mm. Hg and200 mm. Hg (e.g. 100 mm. Hg) or even higher gives advantage comparedwith electrolysis under ordinary pressures. In particular, the pressurefrom a jet pump as described above, run with water of a pressure, forexample, in the range 1 to 10 kg. per cm. and preferably not under 3 kg.per cm. is suitable.

A typical procedure according to the method of the present invention isas follows:

A solution of chlorides of sodium or potassium, preferably saturated, iscontinuously brought into an electric cell in controlled amounts. Thecell is under vacuum to keep the amount of chlorine in the salt solutionlow. By working with the electrolytic cell under reduced pressure onlyvery little of chlorine produced at the anode will go in solution. Themajority will be removed from the cell as a gas. This means that theamount of hypochlorite which is formed in the cell from the chlorine insolution and the alkali from the cathode is small. In the normal cellthe formation of hypochlorite and the reduction of chlorine, at theanode by atomic hydrogen reduce the yield of chlorine. Again, withlittle chlorine in solution there is clearly very little reduction atthe anode. An-

Other advantage of working with the cell at low pressure is the abilityto keep the temperature down. The pressure determines the boiling pointof the electrolyte and hence its maximum temperature, while heatproduced by the electricity in the cell will be taken up by theevaporation of the electrolyte.

The construction of the cell for use in the method of the presentinvention is very simple and may utilise solid electrodes. One preferredcell is one in which the cathode is in the form of a tube around theanode.

To control the amount of salt solution which is fed into the cell anordinary dosing pump can be used but the pressure difference between theatmosphere and the inside of the cell may be utilised. By means of thepressure difference salt solution can be sucked into the cell. The saltsolution is, for example, sucked through an electric magnet valve andthe amount of salt solution through the cell is controlled by theperiods of time when the valve is open and closed. Under short periodsof time in continuous series the valve is open, closed, open, closedetc. These periods are given to the valve by a time relay which can beof transistorized type with thyristor or of a cold cathode valve type.For instance the time of current How can be under a period of second andthe period without current flow can be of one second. By changing theseperiods the amount of salt solution fed into the cell can be varied.

Included in this invention is also the arrangement with this magneticvalve with the time relay to control the amount of salt solution, thecell under vacuum and the pump which can be of either jet pump type orof liquidpiston-type rotary blower.

The electrolytic cell used should be of a type which will permit thegases produced to leave the cell without causing disturbance in thefunction of the cell. Due to the fact that the gases are under vacuumthe volume of the gases is very large in comparison with cells workingunder ordinary pressure or under only slightly reduced pressure. Thiscan cause a pumping action of the electrolyte and may cause theelectrolyte to be present in the cell to a minor degree. The result ofthis is that a higher voltage must be used for the same electricity orelse the effect of the cell is smaller.

To avoid the disadvantage mentioned above the cell can either forinstance be equipped with means for circulation of the electrolyteor/and with electrodes which for instance because of perforation permitthe gases to leave between the electrodes.

One form of the electrolytic cell of the invention may be constructedfollowing:

A rectangular horizontally placed anode of graphite is located under arectangular cathode of iron or an iron containing compound. The cathodeis perforated or otherwise so constructed so that gases may pass throughit. For instance the cathode may be made of thin bars between whichthere is a passage for the gases. Above the cathode is a space for thegases. The inlet and outlet of the cell are located on the short side ofthe rectangle.

In practice the cell is of sandwich construction: the first layer is aplastic plate, the second layer is a packing piece, the third layer isthe graphite anode, the fourth layer is a packing piece, the fifth layeris a plastic frame, the sixth layer is a packing piece and the seventhlayer is a plastic plate.

The layers are held together by means of I-bolts. A holder for thecathode extends through the seventh layer. This holder also forms theconductor between the cathode and the supply of electricity. The holderkeeps the cathode, which is made of perforated iron, in position withits underside close to the topside of the anode. Above the cathode isspace for the produced gases. The inlet to the cell, which for instanceis controlled by a periodically working magnet valve, can be placedanywhere on one short side of the cell. On the opposite short side isthe outlet placed just over the cathode, thus connecting with the spacefor the gases. The out et is connected with a water jet pump which sucksthe complete amount of gases and liquid from the cell and brings it intothe jet. When the cell is being used to produce chlorine for use intreating water the jet water is delivered to the water to bechlorinated.

The chlorination method and apparatus of the present invention appliesnot only to drinking water but to all water which it is wished todisinfect, for example, water discharged from waste water treatmentplants, water in food-manufacturing plants and in other plants where forother reasons the infection of Water must be avoided. Furthermore, theinvention includes processes in which materials dissolved, dispersed orotherwise present in water, for example, cellulose fibres, arechlorinated, the completion of the chlorination being possible tomeasure on the basis of the properties of the treated materials, but inwhich it is the chlorinated products that are to be recovered. In thelatter case especially the chlorination can proceed to a certain degreeof reaction and when that degree is reached, the chlorination may bestopped.

I claim:

1. In a method for the electrolytic production of chlorine whichcomprises introducing a solution of a chloride selected from the groupconsisting of sodium chloride and potassium chloride into anelectrolytic cell, electrolysing said solution of said chloride andremoving chlorine from said cell, the improvement comprising maintainingsaid cell at a pressure not exceeding about 200 mm. Hg during saidelectrolysis to thereby reduce the amount of side reactions occurring inthe system and at the same time eliminate the need for auxiliarycooling.

,2. A method according to claim 1 wherein said cell is kept at apressure of from about 5 mm. Hg to about 200 mm. Hg during saidelectrolysis.

{3. A method according to claim 1 wherein said cell is kept at apressure of from about 1 mm. Hg to about 10 mm. Hg during saidelectrolysis.

4. A method according to claim 1 wherein said cell is kept at a pressureof less than 1 mm. Hg during said electrolysis.

References Cited UNITED STATES PATENTS 1,398,658

HOWARD s. WILLIAMS, Primary Examiner A. C. PRESCOTT, Assistant ExaminerU.S. Cl. X.R. 2049 5, 149

